The present invention relates to an optical scanning system and more particularly, to a system which uses a rotating spinner, having at least one plane linear diffraction grating formed thereon, as the scanning element.
The high brightness characteristics of laser illumination has stimulated interest in sequential optical scanning and has resulted in systems capable of generating high resolution images at high scan rates. These systems have typically used galvanometers, rotating mirrors, acousto-optic or electro-optic elements and rotating holograms as the light spot deflecting elements. Holographic scanning has come to be preferred for many applications due to simplicity of the mechanical geometry, ease and economy of fabrication and higher resolution and scanning speed.
One form of prior art holographic scanner is typically made by holographically forming a plurality of zone-type lenses on the surface of a rotatable disc. The lenses act to focus normally incident reconstruction beams at a locus of focal points which define a desired scan line. Examples of such scanners are found in the systems disclosed by McMahon et al. "Light Beam Deflection Using Holographic Scanning Techniques," Applied Optics, pp. 399-401, Vol. 8, No. 2, Feb. 1969, and U.S. Pat. No. 3,953,105 (Ih). These prior art spinners are subject to certain inherent problems, a principal one being scan line "bow". As the spinner is rotated in the reconstruction mode, the locus of the reconstructed point source is a circle in space. If the image plane is a flat sheet positioned tangentially to the scan circle, the loci of the scan incident on the sheet is, in general, a curved or bowed line. Various solutions have been used to overcome this problem but each has its own drawbacks. A curved image plane can be used in applications where the imaging plane is sufficiently flexible but excluded would be the use of an imaging member such as a xerographic drum. Additional optics can be utilized as proposed by Ih in U.S. Pat. No. 3,953,105 but this type of system is difficult to align and is sensitive to spinner decentration errors.
Another problem is that the spinner is subject to a wobble effect which results in formation of colinear multiple scan lines. This problem has been addressed by the applicant in copending applications, U.S. Ser. No. 708,245 (U.S. Pat. No. 4,239,326) and 921,409 (U.S. Pat. No. 4,243,293) which disclose solutions based on wobble invariance obtained through specific optical geometries. U.S. Pat. No. 4,067,639 and copending application U.S. Ser. No. 921,411 (now abandoned), by the same applicant disclose spinner mounting techniques for reducing wobble.
A third problem not addressed by any of the prior zone-type plate systems, is that of spinner decentration. If the facets (lenses) of a spinner are decentered due to initial fabrication and/or mounting, during the scan mode, the focal position of the facet will oscillate causing output scan distortion.
A fourth problem is chromatic aberration resulting from a wavelength shift in the reconstruction beam, i.e. the image is reconstructed at a wavelength different from the one used for construction. One technique known for compensating for this is the simulated computer holographic lens design programs disclosed by J. N. Latta in his article "Computer-based Analysis of Hologram Imagery and Aberrations", Applied Optics, Vol. 10, No. 3, pp. 599-608, March 1971.
A still further problem encountered when transmission disk spinners are used as the scanning element is the influence of spinner "wedge" on scanner performance. Variations in the thickness of the substrate material, which can occur in the form of localized deviations or of a constant wedge, cause perturbations in the direction of the diffracted beams resulting in colinear multiple scan lines.
Another problem is exposure non-uniformities which may occur either within a scan line (caused, for example, by noise in the reconstruction beam) or from line-to-line (caused by grating-to-grating differences.
It can thus be observed that prior art holographic scanning systems are subject to a multiplicity of problems, many of which can at best be solved on an individual basis leaving other problems still present or can be solved at the expense of worsening existing problems or introducing new ones. In the following description, Applicant discloses a scanning system which by its construction, and playback either eliminates these problems completely or neutralizes their effects.